1. Field of the Invention
The present invention relates to a media agitating mill for performing grinding, mixing, dispersing, homogenizing and the similar actions.
Furthermore, the present invention relates to a method for wet milling ceramic powders into fine particles, especially to submicron particles or finer particles by a media agitating mill.
The "milling" used herein includes preferential grinding which comprises carrying out grinding and mixing simultaneously.
Moreover, the present invention also relates to a fine powder and a method for producing the same and to a method for producing a sintered body using the fine powder, especially to a fine powder of 0.6 .mu.m or less in mean particle size and a method for producing it and a method for producing a sintered body using this fine powder.
2. Description of Related Art
Hitherto, for milling ceramic powders to fine powders, there has been a method which comprises dispersing the ceramic powders in a liquid such as water, ethanol and trichloroethane in a volume of about 10 times or more that of the ceramic powders and agitating the dispersion together with grinding media such as agate, zirconia ceramic, and alumina ceramic, to thereby perform milling of the ceramic powders. It has been reported that in this method, when grinding media of small diameter (in the order of mm) are used, milling time can be shortened as compared with when grinding media of larger size are used. (Tanaka et al, "Zairyo (materials)", Vol. 35, pages 54-58). Recently, media agitating mill which agitates grinding media and powder at a high speed has been noticed as a mill for ceramic powders.
Conventional media agitating mills have a structure comprising at least a milling chamber, grinding media and an agitator and inner face of the milling chamber is made of metals such as stainless steel, ceramics or resins.
It has been said that in order to produce a homogeneous and high density sintered body by ordinary firing, it is essential that the particle size of raw material powder is less than submicron. Recently, a fine powder prepared by a solution method such as a coprecipitation method or alkoxide method has been noticed as raw material powder for obtaining a homogeneous and high density sintered body by firing at a low temperature. Furthermore, as a method for obtaining a fine powder by a milling method, there is a method which uses a media agitating mill which agitates grinding media and powder at a high speed by an agitator.
The conventional methods need a long time for milling ceramic powders to fine powders, especially of a particle size of submicron. As mentioned in the above cited literature, in order to mill a calcined powder of BaTiO.sub.3 to a particle size of less than about 0.6 .mu.m, 100 hours or more is required even if grinding media of 2 mm are used.
The conventional media agitating mills and methods for milling ceramic powders using this mills require a long time for milling ceramic powders to fine powders, especially to a particle size of submicron. Further, in this case, grinding media or an agitator are considerably worn and the components thereof are incorporated into the ceramic powders to cause deterioration and scattering of properties. The milling time can be shortened by increasing the number of revolution or peripheral speed of agitator to increase a milling speed. However, the above-mentioned conventional milling chambers have various defects and the milling speed cannot be increased so much. That is, in case the milling chamber is made of metals such as stainless steel or chromium plating, the milling chamber is considerably worn and the components of the chamber are incorporated into ceramic powders, resulting in deterioration or scattering of the properties. If the milling chamber is made of ceramics such as alumina and zirconia, wear is relatively less than that of the metallic chamber, but is still serious and causes incorporation of components of the chamber into ceramic powders, resulting in deterioration and scattering of the properties. Besides, they are relatively expensive. When the chamber is made of resins such as polyethylene and polyurethane, since they are low in thermal conductivity, heat is considerably generated when a milling speed is increased and thus, the milling speed cannot be sufficiently increased.
According to the conventional solution methods such as a coprecipitation method and alkoxide method, there is obtained a homogeneous fine powder having a particle diameter of from submicron to several nanometers and uniform in particle diameter, but the resulting powder is generally poor in dispersibility. Therefore, a molded body of high density cannot be obtained and hence it is difficult to obtain a homogeneous sintered body because of abnormal growth of particles. Moreover, according to these methods, the composition of the resulting powder is not necessarily the same as that of raw material. Besides, the resulting powder is high in cost.
On the other hand, there is a ball mill method as a milling method for obtaining fine powders and this method has been widely used as a method excellent in mass-producibility. However, this method requires much time for obtaining a powder of submicron in particle size. In addition, a powder prepared by the conventional milling method is inferior in dispersibility and a molded body or sintered body of high density is difficult to obtain.